The present disclosure is directed to detection apparatus for elements. It contemplates the use of NMR techniques. It particularly finds application in an apparatus used in the detection and quantitative measurement of hidden or secreted explosives. The present invention can be readily adapted for detection of other materials in a wide range of circumstances. Explosives are presenting more and more problems to airlines, postal authorities, packing and shipping clerks, and many other people. Explosives in letter bombs and other hidden explosives have created severe problems, causing serious injuries and sometimes death, as radical groups, including political extremists, resort to the use of explosives and secreted bombs to settle their grievances. Detection of explosives hidden in packages, letters and elsewhere has become a problem. They are easy to detect with metal detectors if they incorporate substantial quantities of metal. Known techniques will locate such explosives by detecting the presence of metal. However, it is possible to make nonmetallic explosive bombs. The so-called letter bomb is a good example of this. It is relatively lightweight, not susceptible to conventional metal detection techniques, and is otherwise quite dangerous. It is dangerous because it typically does not explode until someone attempts to open it, resulting in serious injuries to the hands and upper portions of the body. It is so small that it can hardly be detected in and among a typical batch delivery of mail.
The detection technique must obtain a unique indication of the explosive. The unique nature of the response is thus tied to the fact that the primary and basic constituent of any type of secreted bomb or explosive device is, in fact, explosive material itself. As a consequence of the present invention, enhanced nuclear magnetic resonance detection apparatus has been provided which is able to detect explosives without requiring direct physical contact between the suspected material and the detection apparatus. The present invention thus utilizes easily achieved electromagnetic fields which penetrate into the package or suspected parcel. Indeed, the magnetic field will penetrate the earth and other nonmetallic low conductivity materials. Such materials include wood, plastics, glass and the like. For instance, a letter bomb can be easily detected using the present invention. This invention can also detect explosives in baggage or parcels. A land mine which is buried in the ground and which is formed solely of explosives and plastic parts can also be detected. Metal detectors, including beat frequency oscillators, are unable to detect bombs made without metal.
Nuclear magnetic resonance is defined as the resonance achieved whereby energy is transferred between an RF magnetic field and a nucleus placed in a constant magnetic field sufficiently strong to at least partly decouple the nucleus from its orbital electrons. The relationship between frequency at which maximum energy is absorbed by the atomic nuclei of the element, the resonant frequency and the magnetic field intensity is a clue to identification of the particular element involved. The NMR detection technique, in general, is old and is found in various detector systems. The difficulty with it lies in part in the scale factors. For instance, significant quantities of material must be present to concentrate the element of interest so that a sizable response is obtained. The signals obtained by NMR are ordinarily very small which requires high quality detection equipment. To the extent that they are larger for some elements, they typically are smaller for other elements. This is particularly true for some elements where the isotope of the element of interest is available only in minute quantities. Moreover, close coupling has been required to improve the NMR signal.
Explosive detection typically is based on detecting several elements together. Fortunately, the elements to be detected come in different ratios for different explosives which therefore have different signatures. Typically, explosive compounds include hydrogen, nitrogen, carbon and oxygen. The relative amounts of each element varies, and, in some explosives, one of these elements may not be present. The response of hydrogen to NMR techniques is maximum compared to nitrogen. Regretably, hydrogen is typically always a constituent of the materials near or surrounding the specimen of interest. Thus, one may have to look to the responses of several combinations of the elements in the suspected explosive.
The present invention, in the preferred embodiment, uses the transient response to yield enhanced detection and to overcome the problems of a steady state detection apparatus. The problems include a lack of sensitivity in the detector, the difficulties of obtaining adequate magnetic field strength and homogeneity at the suspected specimen, and the difficulty of separating signals from hydrogen nuclei resident in supporting materials such as wood, plastic, soil, etc. The use of transient apparatus reduces the necessity for high quality homogeneous magnetic fields. This lowers the size, cost and complexity of the apparatus. Moreover, since the coupling between nuclei or nuclei and the lattice relates to the relaxation time, the transient NMR signal may be more easily analyzed to delineate hydrogen nuclei in a solid (perhaps the explosive) from hydrogen nuclei in plastic or fluid materials, typically water or pulpy materials such as wood, paper or cloth.
One scale factor which presents great difficulties in NMR techniques utilizing transient or steady state response is the extremely large values of the so-called longitudinal or spin-lattice relaxation time often observed in many compounds. These times can measure tens of minutes, sometimes hours, in solids. Detection of the NMR response from such materials requires that they remain in a polarizing magnetic field, undisturbed for a time comparable to the spin-lattice relaxation time prior to testing and observation. The relaxation time is so unduly large in such materials that NMR detection and measurement cannot be used other than for laboratory investigations. Practical applications are forbidden as a result of this scale factor.
The longitudinal relaxation time (hereinafter referred to as T.sub.1) for selected compounds may be reduced in certain conditions by the present invention. It has been discovered that it is possible to adjust the polarizing magnetic field applied to the specimen of interest so that two atomic elements in the specimen are interacted. As an easy example, consider an explosive material which has nitrogen and hydrogen. It is possible to adjust the polarizing magnetic field so that the separation between Zeeman energy levels for the proton (hydrogen nuclei) coincides with that between the quadrupolar energy levels for the nitrogen spin system. In certain compounds, the hydrogen and nitrogen are situated relative to the lattice such that the hydrogen T.sub.1 is reduced as a result of the transfer of energy between the nitrogen nuclei and the hydrogen nuclei. This transfer is enhanced when the NMR frequency of the hydrogen coincides with the NQR frequency of nitrogen.
The present invention is further capable of discriminating the NMR response of the same type nuclei in a different material. As a simple example, the NMR response of hydrogen nuclei in a solid is typically different from that of hydrogen nuclei in a liquid. As another example, the NMR response of hydrogen in some explosives may be discriminated from that of many nonexplosive materials. This is helpful in discriminating between different types of materials as in the detection of secreted explosives.
The NMR response has a second time constant descriptive of it which is the transverse time response or spin-spin relaxation time constant, or T.sub.2 hereinafter. It has been found highly desirable to seek the longitudinal time response, or T.sub.1, of most elements in contrast to detection of T.sub.2. The present invention is uniquely successful in that it is able to modify and reduce T.sub.1 in selected materials to a smaller value and thereby obtain a more rapid response. This serves to distinguish the NMR response of various materials from other materials. This enables prompt and rapid recognition of the unique signature of various explosive materials.
In an alternate form not using the NMR-NQR between two different types of nuclei, the magnetic field is held steady, and the time between successive NMR responses elicited from the sample is varied. Compounds having different relaxation times T.sub.1 can be discriminated by this form of the invention. For a given element in a particular compound, the response will vary dependent on the elapsed time between successive observations of the response time.